Silicon anamorphic gratings for IR high-resolution spectroscopy with future giant telescopes

Abstract

Future 30m telescopes provide enormous challenges for IR high resolution spectrograph design. The spectrograph collimated beam size will reach ~ 400 mm in order to reach R ~ 25,000 under 0.4 arcsec seeing-limited images. This beam size will push an IR spectrograph volume larger than that of the giant optical echelle spectrograph at 10m telescopes, e.g. the Keck HIRES is 6×6×4 m3 (Vogt et al. 1994). The cost would be enormous considering the entire instrument must be cooled to cryogenic temperatures to be feasible. Here we propose a new kind of IR spectrometer using silicon anamorphic immersion gratings as the main disperser. By operating silicon immersion gratings in an anamorphic immersion mode, the increase in spectral resolving power can be up to a factor of n2 or ~12 times at Brewster’s angle (Dekker 1987). Hence, to reach the same spectral resolution, the collimated beam size is reduced to ~33mm in diameter, which makes the design of the instrument relatively easy. The recent breakthrough in silicon immersion grating technology at Penn State has allowed us to routinely fabricate high quality silicon grisms and immersion gratings with sizes of up to 2 inches, <1% integrated scattered light, and diffraction-limited performance thanks to newly developed techniques. Silicon anamorphic immersion gratings with etched dimensions of ~4 inches are being developed at Penn State. The first grating will be available for testing in late 2002. Currently, industry can supply up to 12 inch diameter silicon ingots. We plan to develop a new tool to handle this large grating size in our state-of-the-art nanofabrication facility. A silicon anamorphic grating of this size can provide a seeing-limited (0.4 arcsec) spectral resolution of R ~ 30,000 or diffraction-limited spectral resolution of R ~ 750,000 at 2.2 microns. In this paper, technical issues related to the design of an anamorphic grating spectrograph are discussed.